Ferrous base manganese age hardening alloy and method



United States Patent 3,392,064 FERROUS BASE MANGANESE AGE HARDENIN ALLOY AND METHOD James Robert Kattus and Joseph D. Morrison, Birmingham, Ala., assignors to Southern Research Institute, Birmingham, Ala., a corporation of Alabama No Drawing. Filed Oct. 13, 1965, Ser. No. 495,687

Claims. (Cl. 148-12.3)

ABSTRACT OF THE DISCLOSURE Ferrous base manganese alloys are disclosed with a method for producing hardened products of the same, the alloys characteristically being age hardenable from a martensitic condition thus having high aging response to attain a high strength level, and in composition the alloys are defined to come within consisting essentially of about 3.0% to 11.0% manganese, approximately 1.5% to about 2.0% silicon, from about 0.6% to about 1.2% titanium, from approximately 0.4% to about 3.4% molybdenum, and the remainder substantially all iron.

This invention relates to ferrous base alloys, contributes new and useful ferrous base manganese alloys, and introduces a method for treating the latter alloys and producing the products.

An object herein is the provision of substantially carbonless ferrous base manganese age hardening alloys which have sutliciently slow age hardening kinetics following solution heat treatment to enable air cooling of the heat treated alloys from the latter temperature to room temperature to be practiced, whereupon the alloys are workable at temperatures in a region including room temperature and before aging, and which alloys have quite satisfactory aging response characteristics and contain a supplemental additive to promote ductility of the alloys within the range of hardnesses obtainable from the age hardening constituents on the aging response.

Another object of the present invention is the provision of wrought products of alloys of the character indicated, taking advantage of the workability of the alloys in the solution heat treated condition, and thereafter hardening and strengthening the Wrought products by aging the alloys by treatment involving temperatures much below the solutioning temperatures.

Another object herein is that of providing alloys of the character indicated which at least in the main, categorically, respond favorably to hot working at temperatures in the vicinity of, and including, the solution heat treating temperature range.

Other objects of the present invention and advantages of the same will be obvious and in part more fully pointed out during the course of the following disclosures.

The invention accordingly resides in the combination of elements, in the compositions of materials, in the several operational steps, and in the relation of each of the same to one or more of the others as described herein, the scope of the application of which is indicated in the claims at the end of this specification.

As conducive to a clearer understanding of certain features of the present invention, numerous ferrous base alloys in the prior art depend upon carbon for hardening and strengthening, this following any fashioning of the alloys in the relatively soft condition and entailing then a high temperature solubilizing heat treatment and quenching. Disadvantages attach to this form of hardening for such reasons as because the products after being fashioned are quite prone to oxidize unless shielded at the severely high temperatures needed for hardening. Also, the heating and hardening effects are of sorts which are "ice likely to bring about excessive warping and other changes bodily, which can little be afforded at the stages of product production operations in which these effects are introduced.

Apart from carbon quench hardening alloys there are numerous heretofore known substantially carbonless age hardening alloys which in general for hardening are preliminarily heated at solution temperature until solid solution is obtained including a phase which later is instrumental to hardening the alloy by coming out of solution during a relatively loW temperature age hardening reheat. Where the alloys support working in the as solution heat treated condition at ordinary low temperatures prior to aging, it will readily be realized that an ensuing age hardening heat treatment at temperatures much below the solution heat treating temperature range avoids the latter severe temperatures, and that hardening by a phase distinctly different from carbon thus is practical.

Leading up to the present invention it has been realized that in alloy with iron, titanium and silicon can produce an age hardening effect, but this has been under the further circumstance that such alloys in the as solutioned heat treated condition prior to aging have been brittle and susceptible to become all the more brittle through aging. Entirely apart from any concept relating to the ferrous base manganese age hardening alloys herein provided, the solubility of manganese in iron at room temperatures has been asserted simply as a general matter by some metallurgists to be close to 1.6% manganese and that if more than the latter amount is added to iron (or to steels very low in carbon), iron and manganese tend to display martensitic-like structures.

An outstanding object of this invention is the provision of ferrous base alloys wherein advantages attendant upon the presence of substantial amounts of manganese are had, and in which alloys silicon and titanium complementally with molybdenum render the alloys industrially practical and highly competitive commercially, the alloys having an adequately low alloy content in view of quite favorable age hardening response characteristics, ductility and workability at room temperatures ir1 the as solutioned heat treated condition before aging, and Worthwhile physical properties after being aged.

Referring now more particularly to the practice of the present invention it is found that by combining proper amounts of iron and manganese in a substantially carbonless alloy system, ferrous base manganese age hardening alloys are bad in which the manganese acts as an austenite stabilizer to the extent that transformation occurs martensitically on cooling the solubilized alloys from a solution heat treating temperature, and the resulting crystalline structure is fine-grained, contributing workability and ductility to the solubilized alloys and contributing toward ductility of the alloys at higher hardnesses subsequently obtained through an age hardening heat treatment of the alloys. Further, it is found that by having proper amounts of silicon, titanium and molybdenum in the aforementioned substantially carbonless alloys system, the silicon and titanium together cause the alloys to have a remarkable age hardening potential, though the silicon and titanium remain stably in solution from the solution heat treating temperature all the way down to much lower temperatures including room tem peratures and until the alloy is thereafter subjected to the age hardening treatment. This is so Whether the solubilized alloy is being air cooled or otherwise is being brought down in temperature such as by quenching in oil or water. Furthermore, the favorable crystalline structure of the alloys together with the molybdenum addition promote ductility into coincidence with increased hardnesses available on the remarkable age hardening a 3 potential of the silicon and titanium constituents of the alloys.

This invention accordingly provides ferrous base age hardening alloys which consist essentially of about 3.0% to about 11.0% manganese, approximately 1.5% to about 2.0% silicon, about 0.6% to about 1.2% titanium, from approximately 0.4% to about 3.4% molybdenum, and the remainder iron or substantially so, permitting the further presence of a supplemental amount of any other constituent or constituents which do not adversely affect the basic characteristics of the alloys and tolerating small amounts of incidental impurities which otherwise could be hurtful such as carbon, aluminum, phosphorus and sulfur. Generally carbon is tolerated up to about 0.06% maximum though preferably in smaller amounts up to about 0.04% maximum, and aluminum, phosphorus and sulfur each generally is tolerated up to about 0.10% maximum. Preferably, however, aluminum does not exceed about 0.05% maximum in the alloy composition and sulfur and phosphorus likewise each does not exceed about 0.04% maximum.

The foregoing quantities of manganese in the alloys are regarded as being controlling to lend workability within a practical range of hardnesses of the alloys in the as solution heat treated condition of the alloys and yet have a crystalline grain structure which will share satisfactorily with the molybdenum toward promoting ductility into coincidence with hardnesses obtained by the age hardening effect of the silicon and titanium upon the alloys when the alloys are hardened by aging heat treatment. The hardnesses of the alloys immediately after solution heat treatment, in preparation for the age hardening treatment, vary upwardly as the amount of manganese is increased from about 3.0% to about 11.0% and with the manganese approaching the latter amount, Working properties are tending to diminish. When the amount of manganese is decreased within the aforementioned range toward the lower limit of the range there is a tendency to encourage embrittlement of the alloys after aging. Alloys therefore having manganese in amounts of about 4.0% to about 10.0% are preferred.

With regard to the hereinbefore noted composition ranges for the age hardening elements, silicon and titanium, an increase in the amounts of both of these elements in their respective ranges enhances the aging response of the alloy, but by having the quantity in the range corresponding to either or both of these elements near to the upper limit of the range there is a tendency toward embrittlement of the alloy through aging, while by having the amount of either or both of these elements in the corresponding range proximate to the lower limit of the range there is a dirninishment of aging response of the alloy. Upon an increase in the amount of either or both titanium and molybdenum to near the upper limit of the respective range noted hereinbefore, hot workability of the alloy, supporting such operations as rolling and forging, is diminished by a tendency toward hot shortness. Through having the molybdenum within the v aforementioned associated range closely approach the lower limit on the amount thereof in the range, the ability of the alloy to promote ductility into coincidence with increased hardnesses available on the hardening potential of the silicon and titanium contents of the alloys is reduced.

The foregoing considerations therefore are regarded as being controlling in achieving such worthwhile properties in the alloys as acceptable freedom from hot shortness, the ability to be formed and fabricated such as at temperatures including room temperature following solution heat treatment and prior to aging, high aging response, and retention of ductility after aging. These properties, while varying in alloys within the more general composition limits on manganese, silicon, titanium and molybdenum noted herein, become all the more favorably pronounced in a preferred alloy composition which in accordance with the present invention contains about 4.0% to about 10.0% manganese, from about 1.5% to approximately 2.0% silicon, titanium in amount of about 0.6% to about 0.9%, from approximately 1.0% to about 2.8% molybdenum, and the remainder substantially iron consistent with other constituents and small amounts of impurities being permissible or tolerated as hereinbefore indicated. Among the latter alloys are importantly those which contain about 1.60% toabout 1.90% silicon and approximately 0.8% to about 0.9% titanium offering enormous strength increases through age hardening.

To harden the present alloys by aging, the alloys first are heated at solution temperature until solid solution with silicon and titanium is thoroughly obtained, the heating most suitably being within a temperature range of about 1775 F. to about 1900 F. for the sake of best aging response of the alloys to occur in a subsequent relatively low temperature heat treatment hereinafter to be described. The solubilizing heat of the alloys is sustained for a long enough period of time for the solid solution to be gained. Ordinarily, a length of time of about onehalf hour after the alloy has fully reached solutioning temperature is adequate particularly where an averag temperature of about 1850 F. is maintained.

Alloys of the present invention, being amenable to hot rolling and to a host of other forms of hot Working at elevated temperatures, are so worked as occasion may demand, thus for example producing such products as forgings, sheet, strip, tubing, rods, bars, I-beams, channels, rails, or the like. The optimum hot working temperatures fall within the solution temperature range. Since the alloys retain silicon and titanium stably in solid-solution, rapid quenching of the alloys from the solution heat treating temperature is not required with the alloys to place them in readiness for age hardening. In fact, alloys here-in are sometimes hot Worked and are directly air cooled from the finishing temperature of the work after concurrently being solubilized as an incident to the heating for hot working. Of course, in other practices still in accordance with the present invention, alloys herein arereheated following hot Working, the reheat being finally to solubilize the alloy at solution temperature, whereupon the alloy is then cooledat a slow rate such as vair or more rapidly by quenching in oil or water to accomplish transformation martensitically and thence on down to about room temperature. The cooled alloy is in readiness for being aged. I

The alloys within the more general composition limits hereinbefore set forth, upon being solution heat treated and cooled to room temperature, are of fine-grained crystalline structure, have ductility and hardness which is considerably short of their full potential on hardness. In view of these properties the alloys are shaped and fabricated at this stage, producing articles and products of the alloys by any one or more such operations as cutting punching, machining, or the like. These operations of course may be performed at room temperature or at other low temperatures which do not materially alter the solution heat treated relatively low temperature condition of the alloys. Thereafter, to harden the alloys, such as the alloy metal in the aforementioned articles and products, the metal is brought up to' aging temperature and there held at an aging heat which endures long enough to bring silicon and titanium out of solution and into dispersed fine form throughout the alloy. The aging heat suitably is within the approximate temperature range of 700 F. to 1000 F. for a length of time depending'upo'n the particular aging temperature or temperatures used. Within this approximate range the particular temperature therefore is not extremely critical, though if temperature within the range is increased the corresponding period of aging time may be decreased. In some instances, for example, the alloys are preliminarily aged at aproximately 750 F. for about one hour and then the aging temperature is raised and the alloys are thus finally aged at about 900 'F. foras long as approximately '4 to 9 hours or more. 'Otheiag'ing practices'irf accdrdancewith"the pfesent invention involve simply heatirig the alloys up to'a temperature of anywhere from 800 F. to about 1000""1"; and maintaining the alloys at the temperature for about3 'to12 hours or more. The aging treatment is terminated by bringing the'aged alloys down to the ehvirons of room temperature from the aging temperafine. This for fexample' is either by air cooling or by niore rapidly quenching such as'in oil or water. 7

The present alloys importantly are stable against being 'overagedz'Their age hardnesses therefore are easily accuratelyaccomplished and even may be curtailed short "or full potential of the alloy on hardness to spare ductility 6r the alloy in the aged condition, in view'of the remarkable aging response of the alloys represented by an increase of at least about 20,000 p.'s.i. 'in each of yield Strength and ultimate tensile strength of the'ag'ed alloys at r'oo'm"temperature"over yield strength 'and ultimate tensile strength of the same alloys in solutioned'and cooled room temperature condition before aging. Retention of ductility through aging and thus'exceeding any particular minimum on ductility is of course not always an essential objective, the other properties of'the aged alloys, including remarkably increased strength brought about by the aging, being quite adequate sometimes for the alloys to serve specialized needs.

'Openhearth furnace production of the-present alloys serves well to meet large tonnage demands, though other furnacing equipment instead is also satisfactory, as for example an electric arc furnace especially where smaller tonnage production is to be accomplished. For maintaining carbon levelwithin the limits desired in the alloy, an oxygen processis found to be advantageous particularly ifhigh-carbon charging stock-is-used and thus initially introduces very-considerable quantities of carbon in the melt. Thus, if need be the electric arc furnace process is adapted to anoxy-process' for adjusting the carbon content to within tolerable amounts as by use of an oxygen lance during refining.

In a typical example of electric furnace production of the alloys, a Herault type direct-arc furnace of about 50,000 pound rated capacity and having a basic lining is initially charged with about 34,000 pounds of low-carbon, low alloy steel scrap counting'therewith iron ore to reduce the carbon content appreciably during carbon boil. Upon completion of the initial charging, the arcs of the furnace arestruck and melt-ing of that portion of the charge next to the electrodes is promoted under relatively low power. After a pool of molten metal is obtained near the electrodes, the power then is increased to accomplish very rapid melt down of the charge:During melt down and during a period immediately following melt down, oxidation of the carbon and other reactive alloying elements occurs. The bathat this point consists essentially of iron.

'Theelectric furnace heating in the present illustrative instance in fa'ct'is carried out along with a double' slagging operation. A slag developed from the furnace charge without any initial additions of CaO to the-furnace is first allowed to become oxidizing, and aftermelt-down of the charge the furnace power is turned off, the electrodes are raised, and the slag is thoroughly raked out. Thereafter, a basic slag'is built up on the melt, this consisting of about 5 parts CaO, 1 part fluorspar, and 1' part sand. The electrodes are lowered, the power turned to a high setting for rapid reheating and a short refining period at high temperature of about 2900" F. to 3000" F. ensues. Then, a sample of the melt is taken for analysis, mainly to determine whether carbon, sulfur and phosphorus contents are within tolerable limits. A satisfactory indication in this respect, enabled by the initial charge and the refinement, is followed by deoxidizing the melt by the addition of approximately 100 pounds of high-purity aluminum. The melt is then ready for a final adjustment in composition and for the latter purpose low-carbon ferro-alloy additionsto the furnace are made and include about 2100 pounds of ferro-manganese of 91% manganese grade, about 800 pounds of ferro-silicon of 76% silicon grade, approximately 1065 pounds of ferro-titanium of 39% titanium grade, and about 2500 pounds of ferro-molybdenum of molybdenum grade. Within about 10 minutes from making these additions, the furnace is tapped into the ladle. The ladle is moved into pouring position, and the melt is poured into molds which in size and shape may depend upon the product or products which are to be produced from the resulting ingots in which the ferrous base manganese alloy is nominally of 5% manganese, 2% molybdenum, 1.75% silicon, 0.75% titanium grade having carbon, phosphorus and sulfur contents each not exceeding about 0.04% maximum and an aluminum content not exceeding about 0.05% maximum.

Satisfactory illustrations of the properties of the present alloys are obtainable through a study of Table I below wherein Alloys 26, 48, 52, and 54 are Within the more general limits on composition hereinbefore set forth, while Alloys 110 and 111 are apart only by having a titanium content which is outside etfective limits for adequate aging response for present purposes. All of the alloys in the table were hot worked at about 1850 F. and were solution heat treated at this same temperature, thereafter air cooled, and then aged in the manner indicated in the table. It will of course be understood that the tabulated amounts of manganese, silicon, titanium and molybdenum are associated with a remainder which is substantially all iron. The further constituents such as carbon, sulfur, phosphorus and aluminum in all of the tabulated alloys were controlled for equivalency of the alloys in this respect, carbon, sulfur, and phosphorus each not exceeding about 0.04% and aluminum not exceeding about 0.05%.

Solution As Aging Aged 0.2% 06- Ult. Tens. Elong. in Alloy Composition Treatment, Solutioned Treatment, Hardness set Yield Strength, 2 in., Bend F./Time Hardness FJTime Rockwell Str., p.s.i. p.s.i. percent Test Rockwell 26 sign/{1 1&4 Si, 0.88 1,850/% hr... 022 85, 800 108, 000 9. 8 Ductile.

0. 5 18 Mn, 1.74 Si, 0 88 1,850/% hr..- C22 750/1 hr.

M 900/4 hrs C37 149, 000 161, 800 7. 0 Do. 5 18 M11, 1.74 Si, 0 88 1,850/% hr-.- C22 750/1 h1.+.

Ti, 1.98 Mo. 900/8% hrs C38 151, 800 164, 000 6. 8 Do. 48 5 1 90 Si, 0 1,5350% hr.-- C20 900/12 hrs 029 Do. 52 5 5T6 lfiuibli? Si, 0 84 1,850/% hr... 020 76, 200 96, 500 9. 5 Do.

1, 5 56 Mn, 1.60 Si, 0 84 1,850/% In... C20 750/1 hr.+

Ti, 0.49 900/6 hrs C38 150, 000 162, 500 1. 0 Some 1biend duct it 54 5.2I0ihg1B21fi5 Si, 1.20 1,850/ hr... 019 75, 000 87, 000 10. 0 Ductile. y

o. 5.20 Mn, 1.85 Si, 1.20 1,850/% hr.-. C19 750/1 ht.+

Ti, 2.02 Mo. /6 hrs C31 109, 000 124, 000 4. 0 Moderate D tilit 3.55 Mn, 1.76 Si, 0.39 1,850/% hr.-- C18 750/1 hr.+ uc y Ti, 1.51 Mo. 900/4 hrs O21 Ductile. 111 10.30 Mn, 1.43 Si, 0.39 1,850/% hr.-- C31 750/1 hr.+

Ti, 2.13 M0. 900/4 hrs 033 Do.

Accordingly it will be appreciated that in the present invention ferrous base manganese alloys and a method are provided wherein the various objects noted together with many thoroughly practical advantages are successfully achieved. The alloys are in fact of reasonably lowalloy content and yet offer such important advantages as being capable of being remarkably enhanced in yield strength and ultimate tensile strengthithrough being solution-heat treated and then aged at temperatures which sensibly are far below solution temperatures, and further it will be appreciated that the alloys lend themselves to air cooling following solution heat treatment and indeed are workable and'capable of being fabricated at or near room temperatures, witha view toward the resulting articles and products subsequently being hardened by heating at aging temperaturesif'the intended purpose of the products and articles so demands that strength thus is to be enhanced. The alloys held at aging temperature for strength enhancement and hardening, accordingly, are at relatively mild temperature as compared with solution temperatures, and so products and articles which are produced from the as solutioned alloys by such steps as those which include cold working and fabricating'are on many occasions types which are spared a re-heat to the more rigorous solution temperature and a re-cooling prior to being aged.

Inasmuch as many embodiments may be made of this invention, and as many changes and modifications may be made of the disclosed embodiments, it will distinctly be understood that the foregoing disclosure is to be considered as illustrative, and not as a limitation.

We claim:

1. A ferrous base manganese alloy capable of responding to an age hardening heat treatment from a solutioned and martensitically transformed cold condition whereupon increases each exceeding about 20,000 p.s.i. in enhancement of yield strength and ultimate tensile strength cor} responding to the latter condition are notable along with increased hardness after the age hardening heat treatment and cooling the alloy to an aged cold condition, said alloy consisting essentially of about 3.0% to 11% manganese, approximately 1.5% to about 2.0% silicon, from about 0.6% to about 1.2% titanium, from approximately 0.4% to about'3.4% molybdenum, and the remainder substantially all iron.

2. A ferrous base manganese alloy as set forth in claim -1, wherein titanium is from about 0.6% to about 0.9%, and molybdenum is from approximately 1.0% to about 2.8%.

3. A ferrous base manganese alloy as set forth in claim 2, wherein manganese is from about 4% to approximately I 4. In the production of a worked and hardened ferrous base manganese alloy product, the art which includes providing a ferrous base manganese alloy consisting essentially of about 3% to 11% manganese, approximately 1.5%-to about 2.0% silicon, from about 0.6% to about 1.2% titanium, from approximately 0.4% .'toabout-3.4% molybdenum, and the remainder substantially all-iron, annealing said alloy in the approximate temperature range of about 1775 F. to 1900-F. and for a periodof time sufiiciently long to provide silicon and titaniumin'solid solution in the alloyifor yield strength andultimate teiisile strength of the alloy at room temperature to-bet in creased by amounts each correspondinglyexceeding about 20,000 p.s.i. through aging, .cooling the'solutioned alloy until transformation occurs martensitically,working the martensitically transformed alloy, and heatingthe resulting product of the worked alloy metal upwardto within an aging temperature range for aperiod oftime long enough and thereafter cooling for'the metalt'a'sag'ed to have increased in yield strength and"ultimate tensile strength by amounts each exceeding'about 20,000 p.s.i. 5'. In the production of a worked and hardened fen rous base manganese alloy product, as "set forth inclair'n 4, wherein the manganese in said ferrous 'ba'se" alloy is from about 4%- to approximately-10%, titaniurn is-from about 0.6% to'about 0.9%, and molybdenum 'is from about 1.0%to about 2.8%. '5

6. In the production of a worked and hardened-ferrous base manganese alloy product, asset forth in claim 4, wherein the solutioned alloy is cooled principallydn air from soluti'oning temperature through transformation which occurs martensitically. A 7. A ferrous'base manganese alloyage hardened by heat treatment from a solutioned conditiomsaid alloy consisting essentially of about 3% 'to' about -1l-% 'manganese, approximately 1.5% to about 2.0% silicon; from about 0.6% to about 1.2% titanium,-f-rom approximately 0.4% to 3.4% molybdenum, and the remainder substarb tially all iron, with said alloy characterized at roomtem perature by a yield strength and an ultimate tensile strength enhanced by amounts each exceeding about 20,000 p.s.i. through aging, correspondingly over yield strength and ultimate tensile strength of the alloy in soldtioned, martensitically transformed, room temperature condition prior to the aging. l 8. A ferrous base manganese age hardened 'alloy as set forth in claim 7, wherein titanium is from about 0.6% to about 0.9%, and mOlybdenum is from approximately 1.0% to about 238%. r

9. Aferrous base manganese age hardened alloy as set forth=in claim 8, wherein-manganese is, from about 4% to approximately 10%. 1 1 10. A ferrous base manganese age hardened alloyas set forth inclaim 7, wherein silicon is from about 1.60% to about 1.90%, and titanium is from about0 .8%to about 0.9% 1

I Nefreferences cited. A

HYLA ND BIZOT, Primary Examiner. I W. W. 'STALLARD, Assistant Examiner. 

